Connected: An Internet Encyclopedia
RFC 1421 - 4.1.1 Types of Keys

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### 4.1.1 Types of Keys

A two-level keying hierarchy is used to support PEM transmission:

1. Data Encrypting Keys (DEKs) are used for encryption of message text and (with certain choices among a set of alternative algorithms) for computation of message integrity check (MIC) quantities. In the asymmetric key management environment, DEKs are also used to encrypt the signed representations of MICs in PEM messages to which confidentiality has been applied. DEKs are generated individually for each transmitted message; no predistribution of DEKs is needed to support PEM transmission.

2. Interchange Keys (IKs) are used to encrypt DEKs for transmission within messages. Ordinarily, the same IK will be used for all messages sent from a given originator to a given recipient over a period of time. Each transmitted message includes a representation of the DEK(s) used for message encryption and/or MIC computation, encrypted under an individual IK per named recipient. The representation is associated with Originator-ID and Recipient-ID fields (defined in different forms so as to distinguish symmetric from asymmetric cases), which allow each individual recipient to identify the IK used to encrypt DEKs and/or MICs for that recipient's use. Given an appropriate IK, a recipient can decrypt the corresponding transmitted DEK representation, yielding the DEK required for message text decryption and/or MIC validation. The definition of an IK differs depending on whether symmetric or asymmetric cryptography is used for DEK encryption:

   1. When symmetric cryptography is used for DEK encryption, an IK is a single symmetric key shared between an originator and a recipient. In this case, the same IK is used to encrypt MICs as well as DEKs for transmission. Version/expiration information and IA identification associated with the originator and with the recipient must be concatenated in order to fully qualify a symmetric IK.

   2. When asymmetric cryptography is used, the IK component used for DEK encryption is the public component [8] of the recipient. The IK component used for MIC encryption is the private component of the originator, and therefore only one encrypted MIC representation need be included per message, rather than one per recipient. Each of these IK components can be fully qualified in a Recipient-ID or Originator-ID field, respectively. Alternatively, an originator's IK component may be determined from a certificate carried in an "Originator-Certificate:" field.

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Connected: An Internet Encyclopedia
RFC 1421 - 4.1.1 Types of Keys